Paypal button (and member login) works for some, not for others - browser?

I set up an paypal donate button for auction tickets which links to paypal and after check out back to wp to a registration form I made which is sent to me. It has worked for some people but now we are getting more and more calls from people who are having problems, eg. paypal button doesn't link anywhere. I informed about browser issues but I can't expect people to work on their browser settings, empty their caches/cookies just so they can donate us. Is there something I can change in the blog to make it work? Thanks for your help as otherwise we will have to take the online payment down.
Same for our members only area. Some people can log in, others can't.